The present invention relates to amalgams for use in fluorescent lamps and other devices which require amalgams.
As discussed in "Amalgams for Fluorescent Lamps" by J. Bloem et al Philips Technical Review 38, P 3-88 1978/79 No. 3, the luminous flux of a fluorescent lamp (low-pressure mercury type) depends to a considerable extent on the mercury-vapour pressure present in the tube. The pressure is determined by the temperature of the coolest part of the tube, which is usually the wall. The maximum luminous flux is reached when the wall temperature is 40.degree. C. which for many fluorescent lamps corresponds to an ambient temperature of 25.degree. C. The wall temperature of lamps in closed luminaires or special lamps can be very much higher. In such conditions a high luminous flux can still be attained by using a suitable amalgam in place of pure mercury. This has the effect of lowering the mercury pressure and also of keeping it more or less stable over a broad temperature range. Bloem et al disclose various amalgams containing mercury and one or more of the metals Pb, Sn, Bi, In, Cd, Ga and Ti and state that of amalgams with three or four metals Bi--Pb--Sn--Hg seemed to be the most promising. Bloem et al discuss Bi.sub.0.53 In.sub.0.47 with 6 atomic percent Hg and Bi.sub.0.47 Pb.sub.0.29 Sn.sub.0.24 with 6 atomic percent Hg, which gave the best results. These amalgams give good stable luminous flux at higher than ambient temperature without the mercury-vapour pressure being too low at ambient temperature thus allowing the lamp to reach its maximum luminous flux.
For a better understanding of the prior art and of the invention reference will be made in the following discussion to the accompanying drawings which shows pressure temperature curves of various amalgams.
U.S. Pat. No. 4,924,142 discloses an amalgam comprising Hg, In, Sn and Zn, wherein the ratio between the atoms of In and Sn is between 3:1 and 8:1; the ratio between the sum of the atoms of In and Sn and the atoms of Zn is between 95:5 and 99:1; and the ratio between the sum of the atoms of In, Sn and Zn and the atoms of Hg is between 95:5 and 99:1. U.S. Pat. No. 4,924,142 discloses an example of the amalgam in which the atomic ratio of the elements In: Sn: Zn is 82.5: 16:1.5, with 2 atomic percent Hg. The temperature pressure curve of such an amalgam is shown by curve 4924142 in the accompanying drawing. The curve shows that the temperature interval of the operating range of the amalgam (the plateau) is between 105.degree. C. and 130.degree. C. where the plateau pressure is about 3.times.10.sup.-3 Torr.
U.S. Pat. No. 4,615,846 discloses an amalgam consisting of 15 to 57 wt % Sn, 5 to 40 wt % Pb, 30 to 70 wt % Bi, 4 to 50 wt % In and 4 to 25 wt % Hg. The temperature pressure curve of such an amalgam is shown by curve 4615846 in the accompanying drawing. The curve shows that within the temperature range 50.degree. to 130.degree. C. the mercury vapour pressure is held at 6.times.10.sup.-3 to 7.times.10.sup.-3 mm Hg (Torr).
EP-B1-0,157,440 discloses an amalgam of Hg and an alloy wherein the alloy is composed of bismuth, lead and silver whereby the mutual ratio of the numbers of atoms of bismuth, lead and silver lies in the quadrangle ABCD of the ternary diagram Bi--Pb--Ag with
A:93% of Bi, 2% of Pb, 5 of Ag; PA1 B:35% of Bi, 60% of Pb, 5% of Ag; PA1 C:35% of Bi, 35% of Pb, 30% of Ag; PA1 D:68% of Bi, 2% of Pb, 30% of Ag (atomic %),
and that the ratio of the sum of the number of atoms of bismuth, lead and silver to the number of atoms of mercury lies between 94:6 and 99:1.
Curve 0157440 on the accompanying drawing shows the pressure temperature curve of such an amalgam where the atom ratio of Bi:Pb:Ag:Hg=53:24:20:3. At about 110.degree. C. the mercury vapour pressure is about 1.5 Pa (11.times.10.sup.-3 mm Hg).
This allows good light output at high temperature, the pressure of 1.5 Pa being maintainable over a range of temperatures as indicated by the plateau about 110.degree. C.